This invention relates to the use of chelating molecules to deactivate iron species to prevent fouling in hydrocarbon fluids.
In a hydrocarbon stream, saturated and unsaturated organic molecules, oxygen, peroxides, and metal compounds are found, albeit the latter three in trace quantities. Of these materials, peroxides can be the most unstable, decomposing at temperatures from below room temperature and above depending on the molecular structure of the peroxide (G. Scott, "Atmospheric Oxidation and Antioxidants", published by Elsevier Publishing Co., NY, 1965).
Decomposition of peroxide will lead to free radicals, which then can start a chain reaction resulting in polymerization of unsaturated organic molecules. Antioxidants can terminate free radicals that are already formed.
Metal compounds and, in particular, transition metal compounds such as copper and iron can initiate free radical formation in three ways. First, they can lower the energy of activation required to decompose peroxides, thus leading to a more favorable path for free radical formation. Second, metal species can complex oxygen and catalyze the formation of peroxides. Last, metal compounds can react directly with organic molecules to yield free radicals.
The first row transition metal species manganese, iron, cobalt, nickel, and copper are already found in trace quantities (0.01 to 100 ppm) in crude oils, in hydrocarbon streams that are being refined, and in refined products. C. J. Pedersen (Ind. Eng. Chem., 41, 924-928, 1949) shows that these transition metal species reduce the induction time for gasoline, an indication of free radical initiation. Copper compounds are more likely to initiate free radicals than the other first row transition elements under these conditions.
To counteract the free radical initiating tendencies of the transition metal species so called metal deactivators are added to hydrocarbons with transition metal species already in the hydrocarbon. These materials are organic chelators that tie up the orbitals on the metal rendering the metal inactive. When metal species are deactivated, fewer free radicals are initated and smaller amounts of antioxidants would be needed to inhibit polymerization.
Not all chelators will function as metal deactivators. In fact, some chelators will act as metal activators. Pedersen showed that while copper is deactivated by many chelators, other transition metals are only deactivated by selected chelators.